Diesel fuel produced from crude oil at petroleum refineries naturally contains almost no oxygen. However, petroleum diesel does naturally contain large amounts of sulfur. Diesel produced from typical crude oils may contain anywhere from 5000 ppm to 20,000 ppm of sulfur. The on-road specification for sulfur in diesel fuel, set by the EPA, is 15 ppm. To remove the excess sulfur in the diesel fuel, refineries use a process known as hydrotreating or hydrodesulfurization, whereby, the diesel fuel is mixed with hydrogen in the presence of a catalyst to chemically remove the sulfur. The by-product of this reaction is hydrogen sulfide (H2S). Most refineries achieve sulfur levels as low as 6-8 ppm.
One of the benefits of using waste greases and tallow to create a fuel is that it naturally contains almost no sulfur, and it has been shown to lower emissions from vehicles. Waste greases and tallow are also alternative fuel sources that may lower the United States dependence of foreign oil and boost the U.S. economy. The U.S. government is interested in further development of bio-fuels for these reasons. The U.S. government has mandated that up to a billion gallons of renewable diesel must be used annually by the year 2012. The State of New Mexico has also passed a mandate that all diesel fuel sold contain 5% renewable diesel blended by 2012.
Fuel producers face several challenges in working to achieve these bio-fuel blends. For example, certain forms of renewable diesel do not meet some of the ASTM (American Society for Testing and Materials) specifications as required by the EPA for on-road diesel. The primary specifications of concern are related to the cold flow properties such as pour point and cloud point. These are the temperatures at which the diesel fuel becomes thick and limits its ability to pour or the temperature at which it begins to become “cloudy.” If a fuel has a high cloud point or pour point, then it will likely gel and plug a fuel filter in cold weather. Biodiesel also falls short of meeting the required specifications in thermal-oxidation stability. The effects of low thermal-oxidation stability are that the fuel will break down as it is heated up or simply oxidize in the presence of air. This will lead to particulate or gum formation, and leave carbon deposits in engines. Fuels with low thermal-oxidation stability also have lower energy values and can lead to poor combustion. For these reasons, it is necessary to improve the quality of bio-fuels so that they can be better integrated into everyday use in the United States.